Osprey (fish hawk)
The scientific name for Osprey is Pandion haliaetus; they are members of the Pandioninae family, with only one species of osprey found worldwide. A unique characteristic about this fascinating raptor is that it is the only raptor that plunges into water for fish.
Ospreys are a medium-to-large sized raptor, measuring 21 to 24.5 inches in length with a 4.5 to 6 foot wingspan and weigh about four pounds. Coloration is black above and white below. The head is mostly white with a black streak through the cheeks. Undersides of the wings and tail are striped black and white with a large black spot at the bend in the wing. Like most raptors, the female is usually larger than the male.
Osprey breeding habitats in North America consists of rivers, lakes, wetlands and coastal marshes. They may build their nest the year before or use the same nest as the previous year. Nests are generally built near the raptor's fledged nest location on dead trees, rock outcrops, buildings, buoys, electric towers, pier pilings, and man-made platforms, almost anywhere near water. Osprey nests are a bulky mass of sticks, often five feet in diameter and two to seven feet thick. They prefer nesting sites that have clear visibility and an abundant food supply. Ospreys seem to form loose colonies. The relative abundance of food determines the number of nests, the proximity of nests and the distance between nests.
Adult male Ospreys arrive at their nesting territories a few days before the female ospreys. Soon after his arrival, the male begins sky-dancing over the nest. During this aerial display, he flies sharply up rapidly beating his wings and often carrying a fish or nesting material. At a height of several hundred feet the bird hovers with tail fanned and talons dangling. He then dives down to varying distances and quickly ascends to repeat the hover several times, often uttering a creee or cheeerk call. The sky-dance display is performed frequently before the arrival of the female osprey, and continues less frequently after her arrival.
The sky-dance is believed to have two functions; it is a territorial display and it advertises for a mate. The return of both birds to a nest appears to be a function of territorial fidelity more than pair bonding.
The arrival of the female is followed by a period of courtship that may last up to three weeks. During courting, the pair spends time on the nest together, as well as mate-feeding and copulating. Soon after the female arrives, the male begins to bring food to her. The female may take the food to a perch to eat, returning after her meal with material to line the nest.
Copulation may take place after the mate feeding ritual and is often preceded by a mating display by the male, the female or both. The female's display posture has slightly drooped wings, horizontal body and tail pointed up and to one side. The male might turn his back to the female, and spread his wings downward, with his tail depressed.
Incubation, which lasts thirty-four to forty days, begins when the first egg is laid. Subsequent eggs are laid one to three days apart, with 2-4 eggs in each clutch. The female usually takes on most of the responsibility of incubation, seldom leaving the nest except to feed. The male will take over incubation until her return.
The first chick to hatch has an advantage over the siblings, which hatch a day or two later. The first chick grows quickly and may dominate the nest. In years when food supply is low, the dominant first chick can usurp the available food supply to the peril of the smaller chicks. This insures survival of at least one of the year's brood. Chicks are brooded by the female for approximately ten days, when they grow too large to fit under her, although she will continue to protect them by covering the young with her wing.
Division of Labor
The male, dedicated to provide for his family, does all the hunting until the chicks can begin to feed themselves, at about six weeks of age. He delivers fish to the female on the nest, who tears off pieces to feed to the young. At three to four weeks of age, the chicks begin to exercise their wings by holding onto the edge of the nest and flapping their wings. The female then moves to a nearby perch to guard the nest. She may leave the nest to hunt when the chicks are six weeks old.
At seven to eight weeks of age the young osprey will take their first flight. They spend days practicing flying and perching, near the male's feeding perch calling when the male returns with food. Two weeks after fledging, the young will start to follow the male on hunting trips. Four to eight weeks after fledging the young Osprey will begin to emulate their parents and hunt fish on their own.
Upon spotting a fish near the surface of the water, Osprey hover almost directly above and then dive nearly vertically toward the fish. At the last moment before impact with the water, the Osprey thrusts its talons forward to snatch the fish. The Osprey is often fully immersed and must struggle to break free of the water and lift the heavy fish into the air. Osprey talons are specially adapted to allow one toe to bend backwards so that there are two claws pointing forward and two in reverse on each foot. This helps secure a good grip on the slippery fish. Scrupulous in maintaining its aerodynamic profile, the osprey adjusts the fish in its grip as soon as it clears the water so that the fish's head is forward and the tail aft. This fishing technique is quite different from that of the bald eagle; they swoop almost parallel to the water surface to grip a fish as it passes over and do not bother to adjust the fish to an aerodynamic position.
In the fall, young Osprey from the northern United States drift south following the coast through Central America to northern South America. Young osprey do not return to North America the following spring. They remain in South America while the adults head north, but will return the following year to their home territory to attract a mate and build a nest. The young pair will not successfully breed until they are three years old. Osprey populations that nest in northeastern North America migrate along the Atlantic coast, crossing the ocean along the West Indies to winter in northern South America. Osprey from the southern states do not need to migrate due to the year-round food supply. They tend to stay and defend their territories.
Threats to Survival
Like eagles and falcons, osprey suffered a major decline in population due to the ingestion of DDT pesticide derivatives in their food supply. These toxins collected in the fatty tissues of fish and other animals and were not expelled from the body. Raptors, at the top of the food chain, collect and retain the poisons accumulated, from single-celled creatures to the creatures that ate them, and so on, up the food chain, to the fish consumed by the Ospreys. The concentration of toxins is amplified in the process. These toxins caused ospreys and other raptors to lay eggs with thin shells that broke during incubation. The United States banned the use of DDT and began to regulate other pesticides; this resulted in a significant recovery of osprey, eagles, and other birds of prey. However these pesticides are still in use in other countries, causing a continued threat from the global food chain.
The sites Osprey select for nest building are exposed to weather and obvious to predators. Storms can destroy exposed nests and predators, from raccoons and opossums to crows, ravens, other hawks, eagles and owls can raid an undefended nest. Due to their preference for exposed sites, Osprey often select the crossbars of power poles as nest locations. This can lead to short circuits when rain soaked nest materials bridge insulators. The birds themselves can be electrocuted if their bodies touch a live wire and a grounded part of a support structure.